1. Field of the Invention
The present invention relates to a mobile communication system. More particularly, the present invention relates to a neighbor cell search method and apparatus of a User Equipment (UE) with the utilization of a Physical Cell Identity (PCI) list of PCIs used by evolved Node Bs (eNBs) for a Closed Subscriber Group (CSG) which is from an eNB in a Long Term Evolution (LTE) system.
2. Description of the Related Art
With the rapid advances of technologies, mobile communication systems have evolved to the 4th Generation (4G) system represented by Long Term Evolution (LTE). LTE has been designed to work under the assumption of various scenarios such as by taking notice of various base station capabilities. For example, LTE has introduced a concept of Closed Subscriber Group (CSG) in which only terminals belonging to a CSG are entitled to access a corresponding CSG cell.
FIG. 1 is a diagram illustrating an architecture of an LTE system according to the related art.
Referring to FIG. 1, the radio access network of the mobile communication system includes evolved Node Bs (eNBs) 105, 110, 115, and 120, a Mobility Management Entity (MME) 125, and a Serving-Gateway (S-GW) 130. The User Equipment (hereinafter, referred to as UE) 135 connects to an external network via eNBs 105, 110, 115, and 120 and the S-GW 130.
In FIG. 1, the eNBs 105, 110, 115, and 120 correspond to legacy node Bs of a Universal Mobile Communications System (UMTS). The eNBs 105, 110, 115, and 120 allow the UE to establish a radio link and are responsible for complicated functions as compared to the legacy node Bs. In the LTE system, all user traffic including real time services such as Voice over Internet Protocol (VoIP) are provided through a shared channel and thus there is a need for a device which is located in the eNB to schedule data based on the state information such as UE buffer conditions, power headroom state, and channel state. Typically, one eNB controls a plurality of cells. In order to secure a data rate of up to 100 Mbps, the LTE system adopts Orthogonal Frequency Division Multiplexing (OFDM) as a radio access technology. Also, the LTE system adopts Adaptive Modulation and Coding (AMC) to determine the modulation scheme and channel coding rate in adaptation to the channel condition of the UE. The S-GW 130 is an entity to provide data bearers so as to establish and release data bearers under the control of the MME 125. The MME 125 is responsible for various control functions and connected to a plurality of eNBs 105, 110, 115, and 120.
FIG. 2 is a diagram illustrating a protocol stack of the LTE system according to the related art.
Referring to FIG. 2, the protocol stack of the LTE system includes Packet Data Convergence Protocol (PDCP) 205 and 240, Radio Link Control (RLC) 210 and 235, Medium Access Control (MAC) 215 and 230, and Physical (PHY) 220 and 225. The PDCP 205 and 240 is responsible for IP header compression/decompression, and the RLC 210 and 235 is responsible for segmenting the PDCP Protocol Data Unit (PDU) into segments in appropriate size for Automatic Repeat Request (ARQ) operation. The MAC 215 and 230 is responsible for establishing connection to a plurality of RLC entities so as to multiplex the RLC PDUs into MAC PDUs and demultiplex the MAC PDUs into RLC PDUs. The PHY 220 and 225 performs channel coding on the MAC PDU and modulates the MAC PDU into OFDM symbols to transmit over radio channel or performs demodulating and channel-decoding on the received OFDM symbols and delivers the decoded data to the higher layer. Also, the PHY layer uses Hybrid ARQ (HARQ) for additional error correction by transmitting 1 bit information indicating for positive or negative acknowledgement from the receiver to the transmitter. This is referred to as HARQ ACK/NACK information. The downlink HARQ ACK/NACK information corresponding to an uplink transmission is transmitted through Physical Hybrid-ARQ Indicator Channel (PHICH), and the uplink HARQ ACK/NACK information corresponding to a downlink transmission can be transmitted through Physical Uplink Control Channel (PUCCH) or Physical Uplink Shared Channel (PUSCH).
A UE can be in one of two states: a connected mode and an idle mode.
Meanwhile, an eNB can be categorized into one of the following types: an acceptable cell, a suitable cell, a barred cell, and a reserved cell. An acceptable cell corresponds to a cell allowing for limited services (emergency call and ETWS), and a cell not barred and satisfying cell selection conditions. A suitable cell corresponds to a cell providing UE with normal service, a cell which is part of selected/registered Public Land Mobile Network (PLMN) or PLMN of the equivalent PLMN list, for a CSG cell, CSG ID broadcast by cell is present in CSG white list, and a cell not barred and satisfying cell selection conditions. A barred cell corresponds to a cell indicated as barred cell in system information. A reserved cell corresponds to a cell indicated as reserved cell in system information.
The UE in the idle mode can be in one of several states according to the type of cell it search for or camps on. For example, when the UE is in the idle mode, the UE may be in a camped normally state, a cell selection when leaving RRC_Connected state, an any cell selection state, and a camped on any cell state.
The camped normally state corresponds to a state in which a suitable cell is found through the cell selection/reselection procedure. In this state, the UE monitors the paging channel to determine whether there is new data to be received from the network and receives system information. The UE also performs measurement for cell reselection and executes a cell reselection procedure.
The cell Selection when leaving RRC_Connected state corresponds to a state in which the UE attempts to search for a suitable cell when it transitions from RRC_CONNECTED state to the RRC_IDLE state.
The any cell selection state corresponds to a state in which the UE attempts to find an acceptable cell of any PLMN when it fails finding a ‘suitable cell’ or is rejected by the selected PLMN, the UE attempts to fine an acceptable cell of any PLMN in this state. When an acceptable cell is found, the UE transitions to the ‘Camped on any cell state.’
The camped on any cell state corresponds to a state in which the UE monitors paging channel to determine whether there is new data to be received from the network and receives system information. The UE also performs measurement for cell reselection and executes cell reselection procedure according to conditions. The UE also perform cell search for adjacent suitable cell.
The UE in idle mode receives system information broadcast by the eNB to acquire the information about the neighbor eNBs. The eNB (hereinafter, interchangeably used with the term ‘macro eNB’) notifies the UEs of the Physical Cell Identifier (PCI) list used by the CSG eNBs through System Information Block (SIB) 4. The PCI list is notified by means of the parameter csg-PhysCellIdRange and, once the csg-PhysCellIdRange is received, the UE camped on a cell of the PLMN assumes that the PCI list is value for 24 hours. If the UE is not a member of the CSG cell for this duration, the cells using the corresponding PCI is ruled out in the cell selection/reselection procedure.
However, the method according to the related art has a drawback in that the PCI used by the CSG eNBs is always ruled out in the cell search (selection/reselection) so as to disturb efficient cell search of the UE.
Therefore, a need exists for a cell search method and apparatus of a UE that is capable of facilitating cell search procedure based on the PCI list used by the CSG cells in adaptation to the UE status.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.